Environmental Engineering Reference
In-Depth Information
12.7.3 Inorganic Analyses
Field parameters Eh and pH were measured at the time of sampling. As
expected in reactive iron barriers, Eh decreased (typically from around −100
to −200 mV), indicating reducing conditions. The pH increased (about two
units), consistently over time at all depths. Monitoring results from 5 to 6 m
bgs levels showed an increase in dissolved iron (mainly ferrous iron) con-
centrations hydraulically downgradient of the pilot-scale barrier. Owing to
the lower pH in the influent groundwater, the pH within the barrier did not
increase to the levels that would be expected to result in significant iron pre-
cipitation. Thus, as a result of the low pH at the Botany site, less iron was
precipitating in the pilot-scale barrier compared to other sites where reactive
iron barriers had been installed. This might be advantageous insofar as less
precipitation should result in less porosity loss in the barrier, but it could
also be disadvantageous if elevated iron concentrations could have esthetic
effects on surface water quality at the groundwater discharge points in the
downgradient Springvale Drain or if aquifer porosity was reduced by pre-
cipitation of iron minerals (likely iron oxyhydroxides) in the downgradient
aquifer.
Sulfate and sulfide concentrations decreased significantly and rapidly (by
one to three orders of magnitude) in the reactive barrier. Sulfide was likely
reacting with iron to form iron sulfide (FeS). Sulfate was being reduced in
the barrier, possibly forming hydrogen sulfide (H
2
S) or FeS. Samples of the
groundwater were analyzed for sulfate-reducing bacteria (SRB) to assess for
possible biological reduction of the sulfate. Very minor populations were
detected in only two samples, suggesting that the sulfate reduction is not
being biologically mediated (Duran et al., 2000).
12.8 Analysis of Iron Cores
The possible formation of FeS suggested that mineral precipitation could be
occurring in the pilot-scale barrier, although, the generally consistent CHC
degradation and hydraulic gradient through the barrier over the 19-month
monitoring period suggested that blinding of reactive sites on the iron par-
ticles and mineral fouling were not occurring to any significant extent. To
evaluate this further, two core samples were extracted from the iron barrier
in August 2000 (month 17) and sent to ETI for analysis.
The core samples were collected by driving a push probe at an angle into
the ground a short distance before the front face of the iron zone and con-
tinuing a short distance into the iron zone. The cores were divided into vari-
ous subsamples to be analyzed for carbonates, sulfur, and other selected
inorganic constituents. Select subsamples were also subjected to RAMAN
spectroscopy and analyzed for microbial activity.
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